1. Field of the Invention
Renewable lignocellulosic biomass is readily available from abundant, inexpensive argicultural residues such as corn stover and straw. Utilization of this material as a carbohydrate source for glucose and ethanol production, and as a metabolic energy source in ruminant feeds, has been severly hampered by the low efficiency with which organisms and enzymes are able to convert the polysaccharide portion of the residue into monomeric sugars. The low conversion efficiency for lignocellulosic materials is the result of two principal factors: (1) unavailability of the cellulose and hemicellulose resulting from the close physical and chemical associations between lignin and these polysaccharides in the plant cell wall, and (2) the degree of crystallinity within the cellulose polymer itself.
Lignin is thought to prevent the degradation of cellulose mainly by acting as a physical barrier between the cellulolytic enzyme and its substrate. Consequently, the rate and extent of enzymatic cellulose degradation in lignocellulosic materials is inversely related to the lignin content, with maxiumum degradation occurring only after 50% or more of the lignin has been removed. Even when lignin levels are low, however, the hydrolysis of cellulose can be limited by the physical properties of the polysaccharide itself. Amorphous regions of cellulose are hydrolyzed at much higher rates than are micro-crystalline regions, for example.
This invention relates to a pretreatment process for rendering the polysaccharide components of lignocellulosic residues available for use in biological systems as sources of carbohydrate.
2. Description of the Prior Art
Numerous pretreatments have been developed in an effort to increase the efficiency of enzymatic saccharification. These processes utilize physical, chemical, and/or biological methods to remove lignin and decrease cellulose crystallinity. Although most of these pretreatments do result in increased cellulose hydrolysis, the yields of glucose obtained are usually still well below theoretical levels. Moreover, processes such as autohydrolysis, alkaline cooking, and steam explosion require substantial energy input in the form of heat and tend to generate toxic side products. A few pretreatments have been developed that allow essentially quantitative conversion of cellulose into glucose, but these processes involve the use of expensive, hightly toxic reagents such as cadoxen, ethylenediamine, or peracetic acid. Toxic constituents in the digest, of course, interfere with subsequent biological saccharification and fermentation steps, and also prohibit use of the digest as an animal feed. Other drawbacks typical of conventional pretreatments include loss of the hemicellulose with the solubilized fraction and also reversion of the cellulose crystallinity upon drying. For example, Dreyfus (U.S. Pat. No. 2,487,114) obtains a product high in alpha-cellulose content and low in hemicellulose by treating straw with hydrogen peroxide in highly alkaline aqueous solution.
The first process to be disclosed in the literature for successfully converting nonwoody lignocellulosic materials into highly digestible products having nearly complete availability of the cellulosic components is that of Gould (U.S. Pat. No. 4,649,113). Gould utilizes a batch process for converting agricultural corp residues to useful carbohydrate sources for ruminants and microbes by treatment with alkaline peroxide. The treatment is under conditions of controlled pH which are designed to conserve the hemicellulose in the recovered carbohydrate fraction; but the process requires relatively large amount of chemical reagents and water, and it generates a stream of liquid waste. Carr et al [Biotechnol. Bioeng. 26: 1252-1257 (Oct. 1984)] treated wheat straw with a variety of chemicals by a continuous process in an extrusion cooker, but found that alkaline peroxide effected essentially no increase in cellulose digestibility when straw at 20% concentration was treated at 97.degree. C. for 5.5 minutes with NaOH (4%, dry straw basis) and H.sub.2 O.sub.2 (2%, dry straw basis).